Present-day manufacturing of semiconductor devices, which includes integrated circuits, liquid crystal displays, thin film magnetic heads, and the like, employs optical exposure apparatus and methods. The increasing degree of integration of such semiconductor devices has placed increasing demands on the optical exposure apparatus to achieve higher levels of resolution. The resolution of an optical exposure apparatus can be approximated by the relation EQU R=k.times..lambda./NA.sub.P
wherein, R is the "resolution" or resolving power of the optical exposure apparatus (i.e., the size of the smallest feature that can be printed), .lambda. is the wavelength of the exposure light, NA.sub.P is the numerical aperture of the projection lens included therein, and k is a constant which depends upon on the type of recording medium used and the process for developing the images formed therein.
To keep up with the increasing degree of integration of semiconductor devices, continuing efforts are being made, as can be understood from the above formula, to increase the resolution of the optical exposure apparatus by shortening the wavelength of the exposure light and/or increasing NA.sub.P. In recent years, KrF krypton fluoride) excimer lasers having an output wavelength of 248 nm have been used for the exposure light source. Moreover, projection lenses NA.sub.P of 0.6 or greater have been commercialized, and features as small as 0.25 microns have been realized.
More recently, in an effort to increase resolution ArF (argon fluoride) excimer lasers having an output wavelength of 193 nm have been gaining attention as a successor light source to KrF excimer lasers. This reduction in wavelength could, in principle, allow the printing of features 0.18 microns or less. However, optical exposure apparatus operable at deep ultra-violet ("DUV", i.e., less than 200 nm) wavelengths are difficult to realize. One reason for this is that in this wavelength region, the materials available for the necessary optical components are currently limited to quartz and calcium fluoride (fluorite). For these materials to be suitable for use in DUV optical exposure apparatuses, they must have sufficient transmittance and internal uniformity (an internal transmittance of 0.995/cm or greater has been achieved with fused quartz, and negligible levels of absorption have been achieved with calcium fluorite). Also, optical components made from these materials require an anti-reflection coating on their surfaces when used at DUV wavelengths, to increase light transmission.
However, even with anti-reflection coatings and minimum levels of absorption, the optical characteristics of fused quartz and calcium fluorite can change due to the heat generated by surface contaminants, which absorb DUV light. Also, moisture and organic substances in the air easily adhere to the lens surfaces of lenses used in the optical exposure apparatuses discussed above. The lens surfaces can be contaminated by these contaminants during the manufacturing of the optical exposure apparatus, as well as during its maintenance. In particular, since these contaminants strongly absorb light having a wavelength of less than 200 nm, transmittance in optical exposure apparatus that use exposure light of less than 200 nm is reduced due to such contaminants adhering to the lens surfaces. For instance, it has been discovered that the transmittance of optical components made from fused quartz and calcium fluoride drops rapidly when exposed to moisture or organic compounds. The amount of this absorption, which can reach up to 0.01 per lens surface, is large compared to the absorption due to the material itself or the surface anti-reflection coatings. Therefore, it is necessary to keep the surfaces of fused quartz or calcium fluorite optical components free of such contaminants.
Japanese patent application Kokai No. Hei 7-294705 discloses a technique relating to a method for photo-cleaning individual optical components with light (hereinafter, "photo-cleaning"). In the photo-cleaning technique disclosed therein, the contaminants adhering to the lens surfaces, in the manner described above, separate from the lens surfaces when irradiated with ultraviolet light, effectively cleaning the lens surfaces. Further, when exposure light of less than 200 nm used in the optical exposure apparatus is ultraviolet light, the contaminants adhering to the lens surfaces may be photo-cleaned by operating the optical exposure apparatus and irradiating the lenses of the optical system with the exposure light. However, this technique does not disclose a method for photo-cleaning all, or the essential optical parts of, the optical components of an optical exposure apparatus after the apparatus has been assembled. It has been discovered by the present inventors that the temporary photo-cleaning of individual optical components by exposing them to DUV light actually facilitates the later absorption of ambient moisture and organic compounds onto the surfaces of the optical components. Consequently, even if individual optical components are photo-cleaned using DUV light, it is extremely difficult to assemble the optical components to form a projection exposure system, and then isolate those components completely from moisture, organic compounds, and other contaminants. This has been a major impediment in realizing a robust DUV projection exposure system.
Nevertheless, the numerical aperture NA.sub.I of illumination optical systems used in projection exposure apparatuses are generally smaller than NA.sub.P of projection lens. Consequently, only one part of the NA region of a projection lens (i.e., the region corresponding to NA.sub.I) is illuminated if the illumination light from the illumination optical system impinges directly onto the projection lens. The result is that only the illuminated region is photo-cleaned. This is problematic because cleaned regions having high transmittance and contaminant-adhered regions having low transmittance arise on the lens surfaces, resulting in unevenness (i.e., non-uniformity) in the amount of light in the mask pattern image. This unevenness is due to transmittance unevenness and degradation of the resolving power caused by a reduction in the effective NA of the projection lens, which causes a drop in the imaging performance of the projection exposure apparatus. This has been another major impediment in realizing a robust DUV projection exposure system.